1. Field of the Invention
The present invention relates to a liquid droplet diameter random variable recording method for an on-demand type ink jet recording head for recording characters and images by discharging ink droplets instantaneously as required. Particularly, the invention relates to a liquid droplet diameter random variable recording method wherein a TJ (thermal jet) recording head having a plurality of heat generating elements on a recording head substrate and nozzles corresponding to these heat generating elements is provided to record by use of a specific ink and transfer sheet and also by varying the volume of ink droplets discharged from each of the nozzles at random with time. The invention also relates to an apparatus using such a method and ink composition which enables the liquid droplet diameter to be varied at random.
2. Related Background Art
FIG. 2 is a view schematically showing the structure of an example of an ink jet recording head of a recording apparatus of the kind according to the prior art. A recording head 21 has a plurality of nozzles (discharging ports) 2 and heat generating elements 3 corresponding to each of the nozzles 2. The total number of the nozzles of the recording head 21 shown in FIG. 2 are 2,048 which are arranged in the direction perpendicular to a surface of a sheet. The heat generating elements 3 transduce a part of applied electrical energy into ink discharging energy. The recording of characters and images is performed as given below. Each of the heat generating elements 3 is selectively driven by a head driving circuit 4 as required for generating heat. Due to this heat generation, bubbles are generated in a pressuring chamber 9. With the development of the bubbles, the ink droplets 5 are forwardly ejected (discharged) from the nozzles 2. The discharged ink droplets fly and are shot onto the surface of a recording sheet which is a recording medium 11 to form desired characters or images. In this respect, a reference numeral 7 designates a head; 8, a protective film to protect the heat generating elements 3; 17, lead lines; and 20, ink in an ink supply tube 19.
The volume Vd of ink droplets discharged from each of the nozzles of an ink jet head having a plurality of ink discharging nozzles has not been completely even, and an average volume Vdm of the ink droplets from each of the nozzles has hitherto been different per nozzle. Also, the volume of the ink droplets discharged from a specific nozzle fluctuates within a regular minute range.
Here, FIG. 3 is a conceptual view showing the "frequency" against "the area of shot ink" when the shot ink on a recording medium has colored the recording medium using a conventional recording head, ink, and transfer sheet. For explanation, FIG. 3 illustrates a case where three nozzles are employed, for example, and the mean value of the shot areas of the ink discharged from each of the nozzles is A for relatively small, B for medium, and C for relatively large. In practice, if the shot ink on the recording medium is continuous, it is difficult to measure the area per droplet exactly. However, with a particular attention given to the fact that the volume of the discharged ink droplet and the shot area has a close correlation, the volume Vd of the ink droplet is measured with the following result.
FIGS. 4A, 4B, and 4C show the data on the measurement of the frequency (%) of the volume Vd of the ink droplets with attention given to the three nozzles a, b, and c of the 2,048 nozzles of a recording head, respectively. The nozzle a represents the characteristics of the 253rd bit; b, 254th bit; and c, 255th bit. In this case, the mean volume Vdm of the ink droplet of the nozzle and standard deviation Vd.sigma. corresponding to each bit are as shown in the following table; where the unit is pl (picoliter) and figures shown in () are the numerical value of %:
TABLE 1 ______________________________________ (a) (b) (c) ______________________________________ bit 253 254 255 Vdm (pl) 162 166 186 Vd.sigma. (pl (%)) 7 (4.3) 8 (4.8) 9 (4.8) ______________________________________
As described above, it is extremely difficult in practice to equalize the mean values Vdm of the volume of the ink droplets discharged from a plurality of nozzles of the kind because the volume Vd of the discharged ink droplets 5 is caused by the configuration and structure of the nozzles 2 and minute dimensional errors brought about by its fabrication to deviate from a target value considerably. For example, there are variations without exception in the width, height, cross-sectional area of the nozzles 2, the configuration of nozzle ends, the distance from the nozzle ends to the heat generating elements 3, and the calorific values and others generated by each of the heat generating elements 3 even when the same electrical energy is equally provided for each of them. Therefore, these variations affect the volume Vd of the ink droplets 5 discharged from each of the nozzles 2.
These variations cause the generation of unevenness in recording in the same direction as the one in which the recording head 21 and recording sheet 11 are shifted correlatively (indicated by an arrow) when a recording is executed as the specimen of a recorded image shown in FIG. 5, for example. FIG. 5 represents the result of a recording using the 251st to 257th bits. The recorded dot diameter of the 253rd bit is slightly smaller while that of the 255th bit is slightly larger in this recording. This recording unevenness is not easily noticeable in a pattern such as characters or the like for which the recording frequency of each of the nozzles 2 (printing ratio) is low. However, when the same pattern is continuously recorded so that the printing ratio is heightened, this kind of unevenness tends to occur. As far as these recording unevennesses remain invisible, there will be no problem. In reality, however, if there is a variation in the areas of a recording point adjacent to each other, which are colored by the ink discharged on a recording medium 11, the recording unevenness becomes visible, leading to the degradation of recording finish (quality).
Particularly, when a recording head 21 of the kind is mass produced, these unevennesses result in reducing the throughful of the recording heads 21 satisfactorily usable for an actual recording so that a cost therefor rises.
As a counter measure, there is a method wherein a means is provided to adjust the electrical energy given to the heat generating elements 3 of each nozzle 2 at the time of discharge in order to prevent a recording unevenness of the kind from being generated. More specifically, the voltage value and/or the pulse width of electrical energy to be applied to the heat generating elements 3 of each nozzle 2 is adjusted so that the volume Vd of the ink droplets 5 can be equalized when discharged. However, unless the number of the nozzles 2 is several, this regulating adjustment method requires a complicated circuit, and particularly when the number of nozzles increases up to such as 24, 48, 64, . . . , . . . , 2,408, or still more, it is desirable to adopt some simpler method.
On the other hand, there is known a method wherein before an electrical signal for the ink discharging induction, some other signal is applied to controlling the magnitude of energy given to the heat generating elements 3 in order to adjust the volume Vd of the ink droplets 5 or to perform a gradient recording, but a method wherein the ink discharging volume Vd from a specific nozzle is intentionally varied at random is not known at all.
Now, FIGS. 6A, 6B, and 6C show the data on the measurements of frequency (%) of the volume Vd of ink droplets with attention given to three nozzles d, e, and f of 2,048 nozzles of another recording head. Here, the reference mark d designates the nozzle characteristics of the 253rd bit; e, 254th bit; and f, 255th bit. In the following table 2, the mean volume Vdm and the standard deviation Vd.sigma. are shown in the same manner as the data shown in the table 1:
TABLE 2 ______________________________________ nozzle d e f ______________________________________ bit 253 254 255 Vdm pl 147 166 184 Vd.sigma. pl (%) 5 (3.4) 4 (2.4) 4 (2.2) ______________________________________
In this respect, the measurement data shown in FIG. 4 are the values when the recording head is driven at 800 Hz. The composition, surface tension, viscosity of the ink used here (comparison ink 16) are shown in the table 3 given below.
TABLE 3 ______________________________________ C. I. direct yellow-86 2 parts diethylene glycol 15 parts isopropyl alcohol 4 parts water 79 parts surface tension: 49 dyn/cm, viscosity: 1.8 cP ______________________________________
Conventionally, as a surface sizing agent of a transfer agent for a transfer sheet used for a copying machine, printer, and the like of an ink jet and thermal jet recording type, and electronic photographing type, an inner sizing agent, gelatin, starch, and the like are used in general; the starch occupies must of its composition.
The ratio of this starch should desirably be 1.0% or more in weight in general for the ink jet and thermal jet recording type. Also, for the electronic photographing type, most of them are approximately 0.1 to 1.5%. It has been necessary to use the above-mentioned transfer materials of the two kinds depending on the optimal properties of the copying machine, printer, and the like of both types. As described earlier, the volume of ink droplets discharged from a plurality of nozzles fluctuates in a certain range, and also, the ink droplets discharged from a specific nozzle fluctuate within a regular range. This is fundamentally due to the fact that ink is discharged by controlling the bubbles in the TJ recording method. It is conceivable that because of the residual amount of the minute air bubbles of the last discharging, the next foaming state is caused to change in a continuous discharging. The residual amount of such minute air bubbles is not constant. It changes each time. Therefore, even the ink droplets from a specific nozzle also fluctuate within a regular range.
Of these conditions, the variation of the mean value Vdm of the volume of the former ink droplets particularly generates the recording unevenness in the same direction as the direction in which the recording head 21 and the sheet 11 are shifted correlatively in operating a recording as shown in FIG. 7. The direction of the correlative shift is indicated by an arrow. FIG. 7 represents the results of recording made by use of the 251st to 257th bits. The recorded dot diameters are: slightly small for the 253rd bit; mean value for the 254th bit; and slightly large for the 255th bit.
This recording unevenness is not easily visible in a pattern such as characters having a low recording frequency (printing ratio) of each nozzle. However, the unevenness tends to be generated when the same pattern is recorded continuously at a higher printing ratio. Although there is no problem as far as the recording unevenness remains invisible, but, in reality, it is visually judged as a recording unevenness if there is a variation in the areas of recording point adjacent to each, which are colored by the ink discharged on a recording medium.